Bone repair is a critically important process due to the amount of skeletal trauma that occurs in our society. Delayed or deficient bone repair is a major morbidity associated with skeletal trauma and results in costly additional treatments which are frequently invasive. While two cyclooxygenases have been characterized that have important functions in numerous inflammatory and normal processes, their role in bone repair remains uncertain. This issue is particularly important since NSAIDs, which inhibit cyclooxygenase activity, are one of the most frequently used drugs in our society and are often advocated for use in individuals undergoing bone repair. Our hypothesis is that cartilage repair is dependent upon by cyclooxygenase activity at three major stages of endochondral bone repair: 1) chondrogenesis, 2) chondrocyte maturation and hypertrophy, and 3) chondrocyte terminal differentiation. Our hypotheses are based upon strong preliminary data that show 1) COX-2 expression in chondrocytes; 2) decreased chondrogenesis following COX-2 inhibition; 3) PGE2 mediated signaling effects in growth plate chondrocytes similar to those induced by PTHrP; 4) Regulation of chondrocyte phenotype/gene expression by PGE2; 5) alterations in chondrogenesis and chondrocyte differentiation in Cox-2 -/- mice; and 5) delayed fracture healing in COX-2 -/- mice. The goal of this study is to further characterize the differential role of COX-1 and COX-2 in endochondral bone formation and the mechanisms that is involved in this process. In specific Aim 1: we will examine the role of cyclooxygenase in chondrogenesis, which is a necessary step in endochondral bone formation and reparative processes using embryonic limb mesenchymal cells derived from wild type, COX-1-/-, and COX-2 -/- mice. In specific Aim 2, we will examine the role of cyclooxygenases in chondrocyte maturation and explore the signaling pathways involved. In specific aim 3 two complementary in vivo models will be used to investigate the role of cyclooxygenases in reparative bone formation: i) an ectopic bone formation model that requires chondrogenesis; and ii) a fracture healing model. In all, the proposal contains state of the art molecular methods to comprehensively investigate a critically important issue with a high degree of clinical relevance.